1. Field of the Invention
The present invention relates to an organic light emitting diode display device, and more particularly to an organic light emitting diode display device that is adaptive for changing a gamma reference voltage step by step in accordance with an image brightness of a current frame, and a driving method thereof.
2. Description of the Related Art
Recently, there have been developed various flat panel display devices reduced in weight and bulk that is capable of eliminating disadvantages of a cathode ray tube. Such flat panel display devices include a liquid crystal display (hereinafter, referred to as “LCD”), a field emission display (hereinafter, referred to as “FED”), a plasma display panel (hereinafter, referred to as “PDP”), and an electro-luminescence (hereinafter, referred to as “EL”) display device, etc.
The EL display device among the flat panel display devices is a self-luminous device which radiates a fluorescent material by a re-combination of an electron and a hole. The EL display device is largely classified into an inorganic EL display device which uses an inorganic compound and an organic EL display device which uses an organic compound depending upon the fluorescent material. Since such an EL display device has been highlighted as a post-generation display owing to its advantage of a low voltage driving, a self-luminous, a thin profile, a wide viewing angle, a fast response speed, and a high contrast, etc.
The organic EL display device is comprised of an electron injection layer, an electron transport layer, a light emitting layer, a hole transport layer, and a hole injection layer. Herein, the electron injection layer is disposed between a cathode and an anode. In the organic EL display device, if a predetermined voltage is applied between an anode and a cathode, an electron which is generated from a cathode moves toward a light emitting layer via the electron injection layer and the electron transport layer, and a hole which is generated from an anode moves toward a light emitting layer via the hole injection layer and the hole transport layer. Thus, an electron and a hole which are supplied from the electron transport layer and the hole transport layer are re-combined to generate a light in the organic light emitting layer.
A circuit configuration of each pixel which is formed at an organic light emitting diode display device of the related art using an organic EL will be described with reference to FIG. 1.
FIG. 1 is an equivalent circuit diagram showing a pixel which is included in an organic light emitting diode display device of the related art.
Referring to FIG. 1, each pixel of the organic light emitting diode display device includes a switch transistor S_TR1, a storage capacitor Cst, an organic light emitting diode OLED, and a driving transistor D_TR1. Herein, The switch transistor S_TR1 is turned-on by a scanning pulse which is supplied via a gate line GL to switch a data voltage which is supplied via a data line DL. The storage capacitor Cst charges a data voltage which is supplied via the switch transistor S_TR1. The organic light emitting diode OLED is turned-on by a driving current which is supplied from a power terminal to which a high potential power voltage VDD is applied to be radiated. The driving transistor D_TR1 is turned-on by a data voltage which is supplied via the switch transistor S_TR1 or a charged voltage of the storage capacitor Cst to drive the organic light emitting diode OLED.
The switch transistor S_TR1 is a NMOS transistor having a gate electrode, a drain electrode, and a source electrode. Herein, the gate electrode is connected to the gate line GL. The drain electrode is connected to the data line DL. The source electrode is commonly connected to the storage capacitor Cst and the gate electrode of the driving transistor D_TR1. The switch transistor S_TR1 is turned-on by a scanning pulse which is supplied via the gate line GL to supply a data voltage which is supplied via the data line DL to the storage capacitor Cst and the driving transistor D_TR1.
One side of the storage capacitor Cst is commonly connected to the switch transistor S_TR1 and a gate electrode of the driving transistor D_TR1, and the other side of the storage capacitor Cst is connected to a ground. The storage capacitor Cst is charged by a data voltage which is supplied via the switch transistor S_TR1. The storage capacitor Cst discharges a discharge voltage thereof to hold a gate voltage of the driving transistor D_TR1 from a point that a data voltage, which is supplied via the switch transistor S_TR1, is not applied to a gate electrode of the driving transistor D_TR1. Accordingly, although a data voltage which is supplied via the switch transistor S_TR1 is not supplied, the driving transistor D_TR1 is maintained as a turned-on state by a discharge voltage of the storage capacitor Cst for a holding period when is hold by the storage capacitor Cst. Herein, a point that a data voltage, which is supplied via the switch transistor S_TR1, is not applied to a gate electrode of the driving transistor D_TR1 is a point that a gate voltage of the driving transistor D_TR1 is dropped.
The organic light emitting diode OLED has an anode and a cathode. In this case, the anode is connected to a power terminal to which a high potential power voltage VDD is applied. The cathode is connected to a drain electrode of the driving transistor D_TR1.
The driving transistor D_TR1 is a NMOS transistor having a gate electrode, a drain electrode, and a source electrode. Herein, the gate electrode is commonly connected to a source electrode of the switch transistor S_TR1 and the switch transistor S_TR1. The drain electrode is connected to a cathode of the organic light emitting diode OLED. The source electrode is connected to a ground. The driving transistor D_TR1 is turned-on by a data voltage which is supplied to a gate electrode via the switch transistor S_TR1 or a discharge voltage of the switch transistor S-TR1 which is supplied to a gate electrode to switch a driving current which is flowed into the organic light emitting diode OLED to a ground. Thus, the organic light emitting diode OLED is radiated by a driving current which is generated by a high potential power voltage VDD.
The organic light emitting diode display device of the related art, which includes the pixels that have the above-mentioned equivalent circuit, analyzes an image data of a current frame which is inputted from a system to drive the organic light emitting diode OLED and the driving transistor D_TR1 in accordance with brightness of an image as follows.
Referring to FIG. 2A, if an image of a current frame, which is inputted from a system, is a dark color or is partially a dark color, the organic light emitting diode device of the related art drives the organic light emitting diode OLED and the driving transistor D_TR1 in order to generate a predetermined peak brightness. Thus, the organic light emitting diode device of the related art has a problem in that the organic light emitting diode OLED and the driving transistor D_TR1 thereof are damaged.
Furthermore, referring to FIG. 2B, if an image of a current frame, which is inputted from a system, is bright color, the organic light emitting diode device of the related art drives the organic light emitting diode OLED and the driving transistor D_TR1 in order to generate a predetermined minimum brightness. Thus, the organic light emitting diode device of the related art has a problem in that the organic light emitting diode OLED and the driving transistor D_TR1 thereof are damaged.